1. Field of the Invention
The present invention relates to a focus condition detecting device for a camera which detects focus condition of an objective lens of the camera from an object image received passing through the objective lens.
2. Description of the Prior Art
There has been known a focus condition detecting device in which two object images are formed by re-focusing two object light bundles having been passed through a first and second areas of an objective lens being symmetric to each other with respect to the optical axis thereof, a relative position relation between these two object images is calculated due to correlation calculation and then, the shift distance of the focus position from the predetermined focus position and the direction of the shift (namely whether forward or rearward with respect to the predetermined focus position) are obtained due to calculation according to a predetermined algorism. An optical system for the focus condition detecting device of this type is shown in FIG. 6. As shown in FIG. 6, this optical system includes a condensor lens 6 being arranged on the predetermined focal plane set behind the objective lens 2 or on a plane set behind the predetermined focal plane and re-focusing lenses 8, 10 being arranged rearward of the condensor lens 6 and, there are arranged two line sensors 12, 14 on focal planes of the re-focusing lenses 8, 10, respectively. As the line sensors, a linear array of CCD (charge coupled device) elements can used desirably.
As shown in FIG. 7 schematically, two object images re-focused by lenses 8, 10 are formed approached toward the optical axis 18 of the objective lens if an object image is focused forward of the predetermined focal plane, and are formed apart from the optical axis 18 respectively if the object image is focused rearward of it.
When the object image is focused just on the predetermined focal plane, the distance between two points corresponded to each other on the object images re-focused becomes equal to a specified value which is determined by the composition of the optical system. Accordingly, the focus condition can be obtained, in principle, by detecting the distance between a point on the first object image and one point on the second object image corresponding to the former point. In the optical system shown in FIG. 6, this distance is calculated by a correlation calculation means 16 from signal data outputted from line sensors 12, 14.
The camera installing such a focus condition detecting device as mentioned above is used for taking not only a picture of a static object but also a picture of an object moving fast. Accordingly, the focus condition detecting device should have a very short detection time for detecting a focus condition.
Moreover, in a single eye reflex camera, a very high accuracy is required for detecting the focus condition and, accordingly, the focus condition detecting device is required to have a high accuracy about the detection of focus condition in order to adapt to the single eye reflex camera.
In the detection of focus condition with use of the device mentioned above, results obtained by repeating the detection with respect to a same object positioned at a constant distance from the camera do not coincide with each other when the object is moving in a plane vertical to the optical axis of the objective lens, or when the camera is subjected to small vibration by being held by unsteady hands so that they distribute about a center value with a certain fluctuation. In other words, the accurate focus detection data can not be obtained only by a single detecting operations.
This uncertainty in the focus condition detection is considered to be caused by factors as follows; uneveness with respect to characteristic of each element of each line sensor and the alignment thereof, unstability of the processing circuit used, and difficulty in correct detection of signal components having frequencies higher than the spatial frequency determined by the sampling theorem of Nykist due to spatial frequency characteristic of the line sensors determined by the pitch of elements aligned linearly upon measuring the illumination distribution on the line sensor (which corresponds to the brightness distribution of the object) and, also, impossibility of detection of exact illumination distributions on the line sensors due to existance of a blind zone between adjacent elements of the line sensors.
In the Japanese-Laid Open Publication No. 78811/1981, there is proposed an automatic focus adjusting device in which the focus adjustment is carried out according to an average of a plurality of defocus data obtained by repeating the focus condition detections.
However, this device has an essential disadvantage in that it is impossible to carry out a quick focus adjustment since the objective lens is started to drive after the average has been obtained by repeating detection operations while keeping the objective lens stopped.
In order to solve this problem, there have been proposed devices A to C as mentioned hereinbelow.
A: wherein the line sensor is devided into two or more blocks, these blocks are used selectively according to the degree of defocusing and the number of sensor elements (cells) contained in the block used for the zone apart from the in-focus zone is reduced in comparison with that of the block used for in-focus zone. (See, for instance Ishida et al. copending U.S. patent application Ser. No. 570,012 filed on Jan. 10, 1984 and assigned to the same assignee to that of the present application).
B: wherein the number of elements (cells) used for the correlation calculation is switched in order according to the amount of defocus (See U.S. Pat. No. 4,573,784).
C: wherein filtering property upon sampling image data and/or sampling width (or the sampling number) are switched according to the amount of defocus (See U.S. Pat. No. 4,561,749).
According to these devices A to C, calculations of a rough value and a precise value regarding the amount of drive of the objective lens are carried out by changing the element number or the sampling width or number of each line sensor.
This is based on the idea that it should take precedence over the exact calculation of the focus position to move the objective lens quickly to a position near to the focus position according to a rough estimation about the driving amount of objective lens in such a condition that the amount of defocus is too large and, therefore it is only time-consuming to calculate the driving amount of the objecive lens based on sampling data having a high accuracy.